A common approach to control the torque of an electric motor is to connect resistors in the motor circuit and selectively switch the resistors effectively into or out of the circuit. In the case of a.c. motors, a wound rotor induction motor may be used with multiple steps of resistors and contactors connected via slip rings to the rotor field circuit. The contactors are selectively opened or closed to include or exclude the resistors from the circuit and thereby control the amount of rotor current and consequently the amount of torque produced by the motor. In the case of d.c. motors, a series wound d.c. motor may be used and supplied with constant potential d.c. power. Multiple steps of resistors and contactors are placed in the supply portion of the motor circuit and the contactors are opened and closed to effectively include or exclude the resistors from the circuit to thereby control the amount of current flowing to the motor and consequently control the torque produced by the motor.
The above-described types of torque control circuits are presently used in large mining shovels for actuating the tripping mechanism of the door of the dipper of the shovels. In these applications, a low current level is supplied utilizing the circuits described above to maintain a low motor torque output adequate to maintain a tight trip line but less than required to trip the dipper door open. When it is desired to actuate the dipper door trip mechanism, resistors in the motor circuit are effectively switched out of the circuit to increase the current level to the motor so that the torque produced by the motor consequently increases to cause actuation of the dipper door trip mechanism.
In motor circuits requiring resistors and contactors to control the torque of the motor, there is considerable power loss in the resistors and consequent heat build-up. In general, the control arrangement is very inefficient. The resistors and contactors require cabinet space and the contactors have frequent duty cycles and thereby have a relatively high wear rate. Also, the current level adjustment is relatively limited by the number of resistors and switching arrangements which are available.